Fix telemetry_perf_unittests_viz

When adding telemetry_perf_unittests_viz I thought the isolate would just run
the targeted script. However it also tries to build a target of the same name.
Since this is an override, there is no target.

This change adds the executable override to the isolate definition.

TBR=dpranke@chromium.org

Bug: 795760
Change-Id: I2482275fffd3cfdee542ddb509f287d37fb75d1a
Reviewed-on: https://chromium-review.googlesource.com/847420Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526497}

testing/buildbot/gn_isolate_map.pyl

@@ -960,6 +960,7 @@
   "telemetry_perf_unittests_viz": {
     "label": "//chrome/test:telemetry_perf_unittests",
     "type": "script",
+    "executable": "telemetry_perf_tests",
     "script": "//testing/scripts/run_telemetry_as_googletest.py",
     "args": [
       "../../tools/perf/run_tests",